The papillary surface epithelium has been proposed to play an important role in the urinary concentrating mechanism. We undertook studies to determine what transporters are present in this epithelium. The papillary surface epithelium was dissected from the surface of the rabbit renal papilla and mounted in a perfusion chamber which allowed both sides to be perfused independently. Cell volume was measured at 25 degrees C using computerized quantitative microscopy which allows continuous measurement of cell volume of the living tissue. Addition of ouabain (0.1 mM) to the basolateral side of the epithelium induced a 20% volume increase. This volume increase was completely inhibited by removal of apical bath NaCl, sodium, chloride, or potassium, but not by removal of urea. As little as 1 nanomolar bumetanide in the apical bath completely inhibited the ouabain-induced cell swelling. Changes in apical osmolality resulting from the addition or removal of NaCl caused cell volume changes that were greater than could be accounted for by osmotic water flow alone. This hyper-response was blocked by bumetanide and was stimulated by vasopressin. These observations are consistent with the presence of Na-K-ATPase on the basolateral membrane and a bumetanide-sensitive, vasopressin-responsive Na-K-Cl cotransporter in the apical membrane. These results are consistent with transpithelial transport of sodium chloride by this epithelium which could modify the composition of the pelvic urine.